Process for preparing sodium alkoxides

ABSTRACT

A process for electrochemical preparation of sodium alkoxide is performed in an electrolysis cell having three chambers, wherein the middle chamber is separated from the cathode chamber by a solid-state electrolyte permeable to sodium ions, and from the anode chamber by a diffusion barrier. The geometry of the electrolysis cell protects the solid-state electrolyte permeable to sodium ions from acidic destruction by the pH of the anolyte that falls in the course of electrolysis. The anolyte used in the process is a brine also comprising carbonates and/or hydrogencarbonates, as well as NaCl. The process solves the problem that CO2 from these carbonates and/or hydrogencarbonates forms in the electrolysis cell during the electrolysis of this brine obtained from pretreatment. The process prevents the formation of a gas bubble in the electrolysis cell that disrupts electrolysis and reduces the contamination of the chlorine with CO2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application No.20165250.0, filed Mar. 24, 2020, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a process for electrochemicalpreparation of sodium alkoxide. The process is performed in anelectrolysis cell having three chambers, wherein the middle chamber isseparated from the cathode chamber by a solid-state electrolytepermeable to sodium ions, for example NaSICON, and from the anodechamber by a diffusion barrier, for example a membrane selective forcations or anions. The geometry of the electrolysis cell protects thesolid-state electrolyte permeable to sodium ions from acidic destructionby the pH of the anolyte that falls in the course of electrolysis. Theanolyte used in the process is a brine also comprising carbonates and/orhydrogencarbonates as well as NaCl. Such brines are typically formed inthe pretreatment of raw brines that are obtained from sea salt, forexample. In this pretreatment, metal ions other than sodium, for exampleSr², Ba²⁺, Ca²⁺, Mg²⁺, are removed from the raw brine by means ofcarbonate precipitation, and hence a brine comprising NaCl and carbonateand/or hydrogencarbonate is obtained. The process according to theinvention solves the problem that CO₂ from these carbonates and/orhydrogencarbonates forms in the electrolysis cell during theelectrolysis of these brines obtained from the pretreatment. It preventsthe formation of a gas bubble in the electrolysis cell that disruptselectrolysis and reduces the contamination of the chlorine, which isespecially obtained in the process according to the invention, with CO₂.

Description of Related Art

The electrochemical preparation of alkali metal alkoxide solutions is animportant industrial process which is described, for example, in DE 10360 758 A1. US 200610226022 A1 and WO 2005/059205 A1. The principle ofthese processes is an electrolysis cell in which the solution of analkali metal salt, for example sodium chloride or NaOH, is present inthe anode chamber, and the alcohol in question or an alcoholic solutionwith a low concentration of the alkali metal alkoxide in question, forexample sodium methoxide or sodium ethoxide, is present in the cathodechamber. The cathode chamber and the anode chamber are separated by aceramic that conducts the alkali metal ion used, for example NaSICON oranalogues thereof for potassium or lithium. On application of a current,chlorine forms at the anode—when a chloride salt of the alkali metal isused—and hydrogen and alkoxide ions at the cathode. The result of thebalancing of charge is that alkali metal ions migrate from the middlechamber into the cathode chamber via the ceramic that is selectivetherefor. The balancing of charge between middle chamber and anodechamber results from the migration of cations when cation exchangemembranes are used or the migration of anions when anion exchangemembranes are used, or from migration of both ion types whennon-specific diffusion barriers are used. This increases theconcentration of the alkali metal alkoxide in the cathode chamber, andthe concentration of the sodium ions in the anolyte is lowered.

NaSICON solid-state electrolytes are also used in the electrochemicalpreparation of other compounds:

WO 2014/008410 A1 describes an electrolytic process for preparingelemental titanium or rare earths. The basis of this process is thattitanium chloride is formed from TiO₂ and the corresponding acid, andthis is reacted with sodium alkoxide to give titanium alkoxide and NaCland finally converted electrolytically to elemental titanium and sodiumalkoxide.

WO 2007/082092 A2 and WO 2009/059315 A1 describe processes for producingbiodiesel, in which, with the aid of alkoxides prepared electrolyticallyby means of NaSICON, triglycerides are first converted to thecorresponding alkali metal triglycerides and are reacted in a secondstep with electrolytically generated protons to give glycerol and therespective alkali metal hydroxide.

The prior art accordingly describes processes that are performed inelectrolysis cells with an ion-permeable layer, for example NaSICONsolid-state electrolytes. However, these solid-state electrolytestypically have the disadvantage that they lack long-term stabilitytoward aqueous acids. This is problematic in that, during theelectrolysis in the anode chamber, the pH falls as a result of oxidationprocesses (for example in the case of preparation of halogens bydisproportionation or by oxygen formation). These acidic conditionsattack the NaSICON solid-state electrolyte to such a degree that theprocess cannot be used on the industrial scale. In order to counter thisproblem, various approaches have been described in the prior art.

For instance, three-chamber cells have been proposed in the prior art.These are known in the field of electrodialysis, for example U.S. Pat.No. 6,221,225 B1.

WO 2012/048032 A2 and US 2010/0044242 A1 describe, for example,electrochemical processes for preparing sodium hypochlorite and similarchlorine compounds in such a three-chamber cell. The cathode chamber andthe middle chamber of the cell are separated here by a solid-stateelectrolyte permeable to cations, for example NaSICON. In order toprotect this from the acidic anolyte, the middle chamber is supplied,for example, with solution from the cathode chamber. US 2010/0044242 A1also describes, in FIG. 6, the possibility of mixing solution from themiddle chamber with solution from the anode chamber outside the chamberin order to obtain sodium hypochlorite.

Such cells have also been proposed in the prior art for the preparationor purification of alkali metal alkoxides. For instance, U.S. Pat. No.5,389,211 A describes a process for purifying alkoxide solutions inwhich a three-chamber cell is used, in which the chambers are delimitedfrom one another by cation-selective solid-state electrolytes or elsenonionic dividing walls. The middle chamber is used as buffer chamber inorder to prevent the purified alkoxide or hydroxide solution from thecathode chamber from mixing with the contaminated solution from theanode chamber.

WO 2008/076327 A1 describes a process for preparing alkali metalalkoxides. This uses a three-chamber cell, the middle chamber of whichhas been filled with alkali metal alkoxide (see, for example, paragraphs100081 and 100671 of WO 2008/076327 A1). This protects the solid-stateelectrolyte separating the middle chamber and the cathode chamber fromthe solution present in the anode chamber, which becomes more acidic inthe course of electrolysis. However, this arrangement has thedisadvantage that the alkali metal alkoxide solution is the desiredproduct, but this is consumed and continuously contaminated as buffersolution. A further disadvantage of the process described in WO2008/076327 A1 is that the formation of the alkoxide in the cathodechamber depends on the diffusion rate of the alkali metal ions throughtwo membranes or solid-state electrolytes. This in turn leads to slowingof the formation of the alkoxide.

The present invention addresses a further problem. This is that it isnecessary in the industrial electrolysis of NaCl to free the salt ofsecondary constituents that it contains on account of its origin asdeposited sea salt before it is subjected to the electrolysis, Theseinclude the removal of insoluble constituents and the precipitation andremoval of ions such as sulfate, Sr², Ba²⁺, Ca²⁺ and Mg²⁺. Althoughthese impurities do not have a direct effect on the electrolysis processin the three-chamber cell, the non-elimination thereof leads to a risein their concentration in the salt solution. The reason for this riselies in the customary construction for electrolyses and the feeding ofsalt into the electrolysis cell. During the electrolysis, there istypically electrochemical conversion of just a portion of the saltdissolved in water. Before the concentration of the salt drops to avalue that would lead to a distinct decrease in conductivity, thesolution is continuously renewed. The residual solution here generallystill contains more than ⅔ of the original concentration of salt. Inorder to make the electrolysis process more economically viable and toprevent large amounts of salt waste from occurring, the brine, afterpassing through the electrolysis, is admixed with fresh salt, such thatthe content in the raw brine obtained thereafter again corresponds tothe concentration that the brine initially had at the start of theelectrolysis in the cell.

This procedure is similar to a closed liquid circuit which is constantlyreplenished with salt. The amount of salt replenished corresponds herein stoichiometric terms to the amount of alkoxide that forms in theelectrolysis.

Constituents that are present in the salt but are not converted in theelectrolysis become concentrated over time with this technology. Thisdoes not impair the conversion in the electrolysis cell immediately, butdoes so after a certain time. For example, side effects such as thedeposition of calcium sulfate in pipelines, which commence atconcentrations in the region of 10 ppm in brines, can lead to problemsin the brine supply, to deposits on the solid-state electrolyte surfaceor to blockage of the ion-permeable diffusion barrier, and stop theelectrolysis process completely or at least affect it in anenergetically unfavourable manner. In addition, this can also lead toreactions at the anode that destroy it.

Specifically when, as is advantageous, the salt used in the electrolysiscomes from sea salt, it is therefore essential to remove secondaryconstituents from the raw brine before it is supplied to theelectrolysis cell for there not to be enrichment of unwanted salts and,in particular, for certain specific limits for the secondaryconstituents not to be exceeded.

As is common knowledge from the operation of chloralkali electrolyses,this depletion proceeds mainly via precipitation reactions resultingfrom addition of carbonate-containing precipitation chemicals orestablishment of particular alkaline pH values at which there iscarbonate and hydroxide precipitation, for example according to thecorresponding reaction equations (1) to (3):

$\begin{matrix}\left. {{FeCl}_{3} + {3\mspace{14mu}{NaOH}}}\rightarrow{{{Fe}({OH})}_{3} + {3\mspace{14mu}{NaCl}}} \right. & (1) \\\left. {{CaCl}_{2} + {{Na}_{2}{CO}_{3}}}\rightarrow{{CaCO}_{3} + {2\mspace{14mu}{NaCl}}} \right. & (2) \\\left. {{MgCl}_{2} + {2\mspace{11mu}{NaOH}}}\rightarrow{{{Mg}({OH})}_{2} + {2\mspace{11mu}{NaCl}}} \right. & (3)\end{matrix}$

The addition of NaOH is achieved via establishment of a particular pH inthe range from 10 to 11 in the brine. Carbonate is generally added witha distinct excess of sodium carbonate (soda). The precipitatedcarbonates and hydroxides are then removed from the brine via afiltration. Performance of this purification step affords a brine havingalkaline pH and a detectable content of carbonates orhydrogencarbonates.

But the presence of carbonates and hydrogencarbonates in the brine inthe electrolysis cell leads to a further problem. During theelectrolysis in the three-chamber cell, the pH of the brine is alteredin the middle chamber and in the anolyte chamber. The pH falls; thebrine becomes more acidic. This effect is attributable to the fact thatchlorine formed at the anode disproportionates directly in the saltsolution according to reaction equation (4):

$\begin{matrix}\left. {{Cl}_{2} + {H_{2}O}}\rightarrow{{HOCl} + {{HCl}.}} \right. & (4)\end{matrix}$

This forms hydrochloric acid and hypochlorous acid. This leads in turnto lowering of the pH of the anolyte brine down to a pH of 3 or lower inthe anolyte chamber. This also lowers the pH in the middle chamber. Thisis certainly the case when, in a three-chamber cell, the middle chamberand anolyte chamber are separated only by the ion-permeable diffusionbarrier through which protons are transported from the anolyte chamberinto the middle chamber.

The reduction in the pH results in release of the carbonates andhydrogencarbonates present in the anolyte as CO₂. In the anolytechamber, this is transported out of the chamber with the chlorineformed. The CO₂ can be separated from the chlorine in downstreamindustrial steps. The steps are common knowledge and industrially triedand tested. Examples of available methods include chlorine scrubbing orchlorine liquefaction. In many applications, a small proportion of CO₂does not disrupt the further use of chlorine in any case. Nevertheless,the contamination of Cl₂ with CO₂ is a fundamental problem, for example,in the treatment of drinking water with chlorine (F. Küke, Vom Wasser2005, 103, 18-22) and is therefore undesirable. In addition, CO₂impurities in chlorine can lead to unwanted reactions, for example aconversion to acid chlorides.

In the case of three-chamber cells, the release of CO₂ is detected onaccount of the reduction in pH in the middle chamber as well, where itlikewise leads to problems. The pH is reduced here in the course ofelectrolysis typically only to values of 8 to 9, i.e. not assignificantly as in the anode chamber. However, this is sufficient tocause the outgassing of CO₂.

By virtue of the fact that the brine is supplied to the electrolysiscell via the middle chamber and thence transferred directly into theanode chamber, CO₂ also accumulates in the middle chamber. While CO₂ canbe removed from the electrolysis cell with the anolyte in the anodechamber, it cannot escape from the middle chamber. The effect of this isthat, after a certain time, the middle chamber is filled with gas to aconsiderable degree. This reduces the brine-covered surface area of thesolid-state electrolyte available for the electrochemical conversion,which increases the resistance of the overall electrolysis. This in turnleads to a reduction in conversion proportional to a reduction in thecurrent between anode and cathode at constant voltage, or to higherenergy costs if the resultant reduction in the amount of current is tobe compensated for by an increase in the voltage between anode andcathode.

It was therefore an object of the present invention to provide animproved process for electrolytic preparation of sodium alkoxide andespecially chlorine, which ensures protection of the cation-conductingsolid-state electrolyte from acid and does not have the aforementioneddisadvantages. More particularly, the process is to feature more sparinguse of the reactants compared to the prior art. In addition, the processis to enable the use of NaCl solution that also includes carbonates andhydrogencarbonates, and the contamination of the chlorine with CO₂ andthe accumulation of CO₂ in the electrolysis cell are to be reduced.

A process which achieves the object of the invention has nowsurprisingly been found.

SUMMARY OF THE INVENTION

The process according to the invention is one for preparing a solutionL₁ <115> of a sodium alkoxide NaOR in the alcohol ROH, especially ofchlorine (Cl₂) and a solution L₁ <115>, in an electrolysis cell E <100>,

-   -   wherein E <100> comprises at least one anode chamber K_(A)        <101>, at least one cathode chamber K_(K) <102> and at least one        interposed middle chamber K_(M) <103>,    -   wherein K_(A) <101> comprises an anodic electrode E_(A) <104>        and an outlet A_(KA) <106>,    -   wherein K_(K) <102> comprises a cathodic electrode E_(K) <105>,        an inlet Z_(KK) <107> and an outlet A_(KK) <109>,    -   wherein K_(M) <103> comprises an inlet Z_(KM) <108> and a gas        outlet G <120>, is separated from K_(A) <101> by a diffusion        barrier D <110> and is separated from K_(K) <102> by a sodium        cation-conducting solid-state electrolyte F_(K) <111>,    -   wherein K_(A) <101> and K_(M) <103> are connected to one another        by a connection V_(AM) <112> through which liquid can be routed        from K_(M) <103> into K_(A) <101>,    -   wherein the process comprises the following steps (a). (b)        and (c) that proceed simultaneously:    -   (a) a solution L₂ <113> comprising the alcohol ROH and        preferably comprising at least one sodium alkoxide NaOR is        routed through K_(K) <102>,    -   (b) a neutral or alkaline, aqueous solution L₃ <114> comprising        NaCl and at least one salt S selected from hydrogencarbonate and        carbonate is routed through K_(M) <103>, then via V_(AM) <112>,        then through K_(A) <101>,    -   (c) voltage is applied between E_(A) <104> and E_(K) <105>,    -   which affords the solution L₁ <115> at the outlet A_(KK) <109>,        wherein the concentration of NaOR in L₁ <115> is higher than in        L₂ <113>,    -   and which affords an aqueous solution L₄ <116> of NaCl and        especially Cl₂ at the outlet A_(KA) <106>, wherein the        concentration of NaCl in L₄ <116> is lower than in L₃ <114>, and        wherein the total concentration of all salts S in L₄ <116> is        lower than in L₃ <114>,    -   and which forms CO₂ <121> in the middle chamber K_(M) <103>,        which is removed from the middle chamber K_(M) <103> via the gas        outlet G <120>,    -   wherein R is an alkyl radical having 1 to 4 carbon atoms.

The invention also includes the following embodiments:

1. Process for preparing a solution L₁ <115> of a sodium alkoxide NaORin the alcohol ROH in an electrolysis cell E <100>,

-   -   wherein E <100> comprises at least one anode chamber K_(A)        <101>, at least one cathode chamber K_(K) <102> and at least one        interposed middle chamber K_(M) <103>,    -   wherein K_(A) <101> comprises an anodic electrode E_(A) <104>        and an outlet A_(KA) <106>,    -   wherein K_(K) <102> comprises a cathodic electrode E_(K) <105>,        an inlet Z_(KK) <107> and an outlet A_(KK) <109>,    -   wherein K_(M) <103> comprises an inlet Z_(KM) <108> and a gas        outlet G <120>, is separated from K_(A) <101> by a diffusion        barrier D <110> and is separated from K_(K) <102> by a sodium        cation-conducting solid-state electrolyte F_(K) <111>,    -   wherein K_(A) <101> and K_(M) <103> are connected to one another        by a connection V_(AM) <112> through which liquid can be routed        from K_(M) <103> into K_(A) <101>,    -   wherein the process comprises the following steps (a), (b)        and (c) that proceed simultaneously:    -   (a) a solution L₂ <113> comprising the alcohol ROH and        preferably additionally comprising at least one sodium alkoxide        NaOR is routed through K_(K) <102>,    -   (b) a neutral or alkaline, aqueous solution L₃ <114> comprising        NaCl and at least one salt S selected from hydrogencarbonate and        carbonate is routed through K_(M) <103>, then via V_(AM) <112>,        then through K_(A) <101>,    -   (c) voltage is applied between E_(A) <104> and E_(K) <105>,    -   which affords the solution L₁ <115> at the outlet A_(KK) <109>,        wherein the concentration of NaOR in L₁ <115> is higher than in        L₂ <113>,    -   and which affords an aqueous solution L₄ <116> of NaCl at the        outlet A_(KA) <106>, wherein the concentration of NaCl in L₄        <116> is lower than in L₃ <114>, and wherein the total        concentration of all salts S in L₄ <116> is lower than in L₃        <114>,    -   and which forms CO₂ <121> in the middle chamber K_(M) <103>,        which is removed from the middle chamber K_(M) <103> via the gas        outlet G <120>,    -   wherein R is an alkyl radical having 1 to 4 carbon atoms.

2. Process according to Embodiment 1, wherein R is selected from thegroup consisting of methyl and ethyl.

3. Process according to Embodiment 1 or 2, wherein the diffusion barrierD <110> is selected from the group consisting of cation-conducting andanion-conducting membranes.

4. Process according to Embodiment 3, wherein the diffusion barrier D<110> is a sodium cation-conducting membrane.

5. Process according to any of Embodiments 1 to 4, wherein the flowdirection of L₃ <114> in the middle chamber K_(M) <103> is the oppositeof the flow direction of L₃ <114> in the anode chamber K_(A) <101>.

6. Process according to any of Embodiments 1 to 5, wherein theconnection V_(AM) <112> is formed within and/or outside the electrolysiscell E <100>.

7. Process according to any of Embodiments 1 to 6, wherein theconnection V_(AM) <112> between middle chamber K_(M) <103> and anodechamber K_(A) <101> is arranged in such a way that at least a portion ofthe aqueous solution L₃ <114> flows completely through the middlechamber K_(M) <103> and the anode chamber K_(A) <101>.

8. Process according to any of Embodiments 1 to 7, wherein the sodiumcation-conducting solid-state electrolyte F_(K) <111> has a structure ofthe formula M^(I) _(1+2w+x−y+z)M^(II) _(w)M^(III) _(x)Zr^(IV)_(2−w−x−y)M^(V) _(y)(SiO₄)_(z)(PO₄)_(3−z),

where M^(I) is selected from Na⁺ and Li⁺,

M^(II) is a divalent metal cation,

M^(III) is a trivalent metal cation.

M^(V) is a pentavalent metal cation,

the Roman indices I, II, III, IV, V indicate the oxidation numbers inwhich the respective metal cations exist,

and w, x, y, z are real numbers, where 0≤x<2, 0≤y<2, 0≤w<2, 0≤z<3,

and where w, x, y, zare chosen such that 1+2w+x−y+z≥0 and 2−w−x−y≥0.

9. Process according to Embodiments 1 to 8, wherein L₂ <113> comprisesthe alcohol ROH and sodium alkoxide NaOR.

10. Process according to Embodiment 9, wherein the mass ratio of NaOR toalcohol ROH in L₂ <113> is in the range from 1:100 to 1:5.

11. Process according to Embodiment 9 or 10, wherein the concentrationof NaOR in L₁ <115> is 1.01 to 2.2 times higher than in L₂ <113>.

12. Process according to any of Embodiments 1 to 11, wherein theconcentration of NaCl in L₃ <114> is in the range from 3.5 to 5 mol/l.

13. Process according to any of Embodiments 1 to 12, wherein theproportion by mass of the salt S in the solution L₃ is in the range from0.1 ppm to 10% by weight.

14. Process according to any of Embodiments 1 to 13, which is performedat a temperature of 20 to 70° C. and a pressure of 0.5 to 1.5 bar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the process according to the invention in a three-chambercell E <100> comprising a cathode chamber K_(K) <102>, an anode chamberK_(A) <101> and an interposed middle chamber K_(M) <103>.

FIG. 2 shows an embodiment of the process according to the inventioncorresponding to that shown in FIG. 1 .

FIG. 3 shows a diagram of the progression of voltage in the electrolysisin a three-chamber cell with and without ventilation of the middlechamber.

DETAILED DESCRIPTION OF THE INVENTION Figures

FIG. 1 shows the process according to the invention in a three-chambercell E <100> comprising a cathode chamber K_(K) <102>, an anode chamberK_(A) <101> and an interposed middle chamber K_(M) <103>. The threechambers are bounded by an outer wall <117> of the three-chamber cell E<100>. The cathode chamber K_(K) <102> is also separated from the middlechamber K_(M) <103> by an NaSICON solid-state electrolyte F_(K) <111>which is selectively permeable to sodium ions. The middle chamber K_(M)<103> is additionally separated in turn from the anode chamber K_(A)<101> by a diffusion barrier D <110>. The NaSICON solid-stateelectrolyte F_(K) <111> and the diffusion barrier D <110> extend overthe entire depth and height of the three-chamber cell E <100>.

A solution of sodium methoxide in methanol L₂ <113> is routed throughthe cathode chamber K_(K) <102>. An aqueous solution of sodium chlorideL₃ <114> with pH 10.5 comprising sodium carbonate is introduced throughthe inlet Z_(KM) <108>, in the direction of gravity, into the middlechamber K_(M) <103>. The connection V_(AM) <112> formed between anoutlet A_(KM) <118> of the middle chamber K_(M) <103> and an inletZ_(KA) <119> of the anode chamber K_(A) <101> connects the middlechamber K_(M) <103> to the anode chamber K_(A) <101>. Sodium chlorideand sodium carbonate solution L₃ <114> is routed through this connectionV_(AM) <112> from the middle chamber K_(M) <103> into the anode chamberK_(A) <101>. On application of a voltage, methanol is reduced tomethoxide and H₂ in the cathode chamber K_(K) <102>. At the same time,sodium ions diffuse from the middle chamber K_(M) <103> through theNaSICON solid-state electrolyte F_(K) <111> into the cathode chamberK_(K) <102>. Overall, this increases the concentration of sodiummethoxide in the cathode chamber K_(K) <102>, which affords a methanolicsolution of sodium methoxide L₁ <115>, the sodium methoxideconcentration of which is elevated compared to L₂ <113>. In the anodechamber K_(A) <101>, chloride ions from L₃ <114> are oxidized to Cl₂.

Cl₂ gives an acidic reaction in aqueous solution. Owing to the geometryof the three-chamber cell E <100> and the guiding of the aqueoussolution L₃ <114>, the acid-sensitive NaSICON solid-state electrolyte<111> is protected from the elevated acidity, compared to L₃ <114>, ofthe solution L₄ <116> that results in the anode chamber K_(A) <101>. Atthe same time, gaseous CO₂ <121> forms in the middle chamber K_(M)<103>, a process that increases as electrolysis progresses. This CO₂<121> can escape via the gas outlet <120> mounted on the middle chamberK_(M) <103>. This avoids a gas bubble of CO₂ <121> in the middle chamberK_(M) <103>.

FIG. 2 shows an embodiment of the process according to the inventioncorresponding to that shown in FIG. 1 . The sole difference here is thatthe connection V_(AM) <112> from the middle chamber K_(M) <103> to theanode chamber K_(A) <101> is formed by a perforation in the diffusionbarrier D <110>.

FIG. 3 shows a diagram of the progression of voltage in the electrolysisin a three-chamber cell with and without ventilation of the middlechamber. The measurement points of the example conducted withoutventilation in the three-chamber cell are represented by the symbol (♦),and those of the example conducted with ventilation in the three-chambercell by the symbol (▪). The x axis shows the time in hours, while the yaxis shows the percentage change in the initial voltage measured involts. The comparison shows that the cell according to the inventiongives a constant voltage progression, whereas the voltage rises rapidlyin the case of the three-chamber cell without ventilation owing to theformation of a CO₂ gas bubble.

Electrolysis Cell E

The process according to the invention is performed in an electrolysiscell E comprising at least one anode chamber K_(A), at least one cathodechamber K_(K) and at least one interposed middle chamber K_(M). Thisalso includes electrolysis cells E having more than one anode chamberK_(A) and/or cathode chamber K_(K) and/or middle chamber K_(M). Suchelectrolysis cells in which these chambers are joined to one another inthe form of modules are described, for example, in DD 258 143 A3, US2006/0226022 A1.

The anode chamber K_(A) comprises an anodic electrode E_(A). A usefulanodic electrode E_(A) of this kind is any electrode familiar to theperson skilled in the art that is stable under the conditions of theprocess according to the invention. These are described, in particular,in WO 2014/008410 A1, paragraph [024] or DE 10360758 A1, paragraph[031]. This electrode E_(A) may consist of one layer or consist ofmultiple planar layers parallel to one another that may each beperforated or expanded. The anodic electrode E_(A) especially comprisesa material selected from the group consisting of ruthenium oxide,iridium oxide, nickel, cobalt, nickel tungstate, nickel titanate,precious metals such as, in particular, platinum, supported on a supportsuch as titanium or Kovar k (an iron/nickel/cobalt alloy in which theindividual components are preferably as follows: 54% by mass of iron,29% by mass of nickel, 17% by mass of cobalt). Further possible anodematerials are especially stainless steel, lead, graphite, tungstencarbide, titanium diboride. Preferably, E_(A) comprises a titanium anodecoated with ruthenium oxide/iridium oxide (RuO₂+IrO₂/Ti).

The cathode chamber K_(K) comprises a cathodic electrode E_(K). A usefulcathodic electrode E_(K) of this kind is any electrode familiar to theperson skilled in the art that is stable under the conditions. These aredescribed, in particular, in WO 2014/008410 A1, paragraph [025] or DE10360758 A1, paragraph [030]. This electrode E_(K) may be selected fromthe group consisting of mesh wool, three-dimensional matrix structureand “balls”. The cathodic electrode E_(K) especially comprises amaterial selected from the group consisting of steel, nickel, copper,platinum, platinized metals, palladium, carbon-supported palladium,titanium. Preferably, E_(K) comprises nickel.

The at least one middle chamber K_(M) is between the anode chamber K_(A)and the cathode chamber K_(K).

The electrolysis cell E typically has an outer wall W_(A). The outerwall W_(A) is especially selected from a material selected from thegroup consisting of steel, preferably rubberized steel, plastic,especially from Telene® (thermoset polydicyclopentadiene), PVC(polyvinylchloride), PVC-C (post-chlorinated polyvinylchloride), PVDF(polyvinylidenefluoride). W_(A) may especially be permeated for inletsand outlets and for the gas outlet <120>. Within W_(A) are then the atleast one anode chamber K_(A), the at least one cathode chamber K_(K)and the at least one interposed middle chamber K_(M).

K_(M) is separated from K_(A) by a diffusion barrier D and from K_(K) bya sodium cation-conducting solid-state electrolyte F_(K).

The diffusion barrier D used may be any material that is stable underthe conditions of the process according to the invention and prevents orslows the transfer of protons from the liquid present in the anodechamber K_(A) into the middle chamber K_(M).

The diffusion barrier D used is especially a non-ion-specific dividingwall or a membrane permeable to specific ions. The diffusion barrier Dis preferably a membrane permeable to specific ions.

The material for the non-ionic dividing wall is especially selected fromthe group consisting of fabric, which is especially textile fabric ormetal weave, glass, which is especially sintered glass or glass frits,ceramic, especially ceramic frits, membrane diaphragms.

If the diffusion barrier D is a “membrane permeable to specific ions”,what this means in accordance with the invention is that the respectivemembrane promotes the diffusion of particular ions therethrough overothers. More particularly, what this means is membranes that promote thediffusion therethrough of ions of a particular charge type over ions ofthe opposite charge. Even more preferably, membranes permeable tospecific ions also promote the diffusion of particular ions of onecharge type over other ions of the same charge type therethrough.

Preferably, the diffusion barrier D is accordingly an anion-conductingmembrane or a cation-conducting membrane. More preferably, the diffusionbarrier D is a cation-conducting membrane.

According to the invention, anion-conducting membranes are those thatselectively conduct anions, preferably selectively conduct particularanions. In other words, they promote the diffusion of anionstherethrough over that of cations, especially over protons: even morepreferably, they additionally promote the diffusion of particular anionstherethrough over the diffusion of other anions therethrough.

According to the invention, cation-conducting membranes are those thatselectively conduct cations, preferably selectively conduct particularcations. In other words, they promote the diffusion of cationstherethrough over that of anions; even more preferably, they promote thediffusion of particular cations therethrough over the diffusion of othercations therethrough, more preferably still that of cations that are notprotons, more preferably sodium cations, over protons.

What is meant more particularly by “promote the diffusion of particularions X over the diffusion of other ions Y” is that the coefficient ofdiffusion (unit: m²/s) of ion type X at a given temperature for themembrane in question is higher by a factor of 10, preferably 100,preferably 1000, than the coefficient of diffusion of ion type Y for themembrane in question.

The anion-conducting membrane used is especially one selective forchloride. Such membranes are known to and can be used by the personskilled in the art.

Salt S is at least one selected from hydrogencarbonate and carbonate,more preferably at least one selected from sodium carbonate and sodiumhydrogencarbonate.

The anion-conducting membrane used is preferably a chloride-selectivemembrane.

Anion-conducting membranes are described, for example, by M. A. Hickner,A. M. Herring, E. B. Coughlin. Journal of Polymer Science, Part B:Polymer Physics 2013, 51, 1727-1735 and C. G. Arges, V. Ramani, P. N.Pintauro, Electrochemical Society Interface 2010, 19, 31-35. WO2007/048712 A2 and on page 181 of the textbook by Volkmar M. Schmidt,Elektrochemische Verfahrenstechnik: Grundlagen, Reaktionstechnik,Prozessoptimierung [Electrochemical Engineering: Fundamentals, ReactionTechnology. Process Optimization], 1st edition (8 Oct. 2003).

Even more preferably, anion-conducting membranes used are accordinglyorganic polymers that are especially selected from polyethylene,polybenzimidazoles, polyether ketones, polystyrene, polypropylene andfluorinated membranes such as polyperfluoroethylene, preferablypolystyrene, where these have covalently bonded functional groupsselected from —NH₃ ⁺, —NRH₂ ⁺, —NR₃ ⁺, ═NR⁺; —PR₃ ⁺, where R is alkylgroups having preferably 1 to 20 carbon atoms, or other cationic groups.They preferably have covalently bonded functional groups selected from—NH₃ ⁺, —NRH₂ ⁺ and —NR₃ ⁺, more preferably selected from —NH₃ ⁺ and—NR₃ ⁺, even more preferably —NR₃ ⁺.

When the diffusion barrier D is a cation-conducting membrane, it is mostpreferably a sodium ion-conducting membrane.

Cation-conducting membranes are described, for example, on page 181 ofthe textbook by Volkmar M. Schmidt, Elektrochemische Verfahrenstechnik:Grundlagen, Reaktionstechnik. Prozessoptimierung, 1st edition (8 Oct.2003).

Even more preferably, cation-conducting membranes used are accordinglyorganic polymers that are especially selected from polyethylene,polybenzimidazoles, polyether ketones, polystyrene, polypropylene andfluorinated membranes such as polyperfluoroethylene, preferablypolystyrene and poly perfluoroethylene, where these bear covalentlybonded functional groups selected from —SO₃ ⁻, —COO⁻, —PO₃ ²⁻ and—PO₂H⁻, preferably —SO₃ ⁻ and —COO⁻, more preferably —SO₃ (described inDE 10 2010 062 804 A1, U.S. Pat. No. 4,831,146).

This may be, for example, a sulfonated polyperfluoroethylene (Nafion®with CAS number: 31175-20-9). These are known to the person skilled inthe art, for example from WO 2008/076327 A1, paragraph [058], US2010/0044242 A1, paragraph 100421 or US 2016/0204459 A1, and arecommercially available under the Nafion®, Aciplex® F. Flemion®,Neosepta®, Ultrex®, PC-SK® trade names, Neosepta® membranes aredescribed, for example, by S. A. Mareev, D. Yu. Butylskii, N. D.Pismenskaya, C. Larchet, L. Dammak, V. V. Nikonenko, Journal of MembraneScience 2018, 563, 768-776.

If a cation-conducting membrane is used as diffusion barrier D, thismay, for example, be a polymer functionalized with sulfonic acid groups,especially of the formula P_(NAFION) below, where n and m mayindependently be a whole number from 1 to 10⁶, preferably a whole numberfrom 10 to 10⁵, more preferably a whole number from 10² to 10⁴.

A useful sodium cation-conducting solid-state electrolyte F_(K) is anysolid-state electrolyte that can transport sodium cations from themiddle chamber K_(M) into the cathode chamber K_(K). Such solid-stateelectrolytes are known to the person skilled in the art and aredescribed, for example, in DE 10 2015 013 155 A1, in WO 2012/048032 A2,paragraphs [0035], [0039], [0040], in US 2010/0044242 A1, paragraphs[0040], [0041], in DE 10360758 A1, paragraphs [014] to [025]. They aresold commercially under the NaSICON name. A sodium ion-conductingsolid-state electrolyte F_(K) more preferably has an NaSICON structure.NaSICON structures usable in accordance with the invention are alsodescribed, for example, by N. Anantharamulu, K. Koteswara Rao, G.Rambabu, B. Vijaya Kumar, Velchuri Radha, M. Vithal, J Mater. Sci 2011,46, 2821-2837.

NaSICON preferably has a structure of the formula M^(I)_(1+2w+x−y+z)M^(II) _(w)M^(III) _(x)Zr^(IV) _(2−w−x−y)M^(V)_(y)(SiO₄)_(z)(PO₄)_(3−z),

M^(I) is selected from Na⁺, Li⁺, preferably Na⁺.

M^(II) is a divalent metal cation, preferably selected from Mg²⁺, Ca²⁺,Sr²⁺, Ba²⁺, Co²⁺, Ni²⁺, more preferably selected from Co²⁺, Ni²⁺.

M^(III) is a trivalent metal cation, preferably selected from Al³⁺,Ga³⁺, Sc³⁺, La³⁺, Y³⁺, Gd³⁺, Sm³⁺, Lu³⁺, Fe³⁺, Cr³⁺, more preferablyselected from Sc³⁺, La³⁺, Y³⁺, Gd³⁺, Sm³⁺, especially preferablyselected from Sc³⁺, Y³⁺, La³⁺.

M^(V) is a pentavalent metal cation, preferably selected from V⁵⁺, Nb⁵⁺,Ta⁵⁺.

The Roman indices I, II, III, IV, V indicate the oxidation numbers inwhich the respective metal cations exist.

w, x, y, z are real numbers, where 0≤x<2, 0≤y<2, 0≤w<2, 0≤z<3,

and where w, x, y, z are chosen such that 1+2w+x−y+z≥0 and 2−w−x−y≥0.

Even more preferably in accordance with the invention, NaSICON has astructure of the formula Na_((1+4v))Zr₂Si_(v)P_((3−v))O₁₂ where v is areal number for which 0≤v≤3. Most preferably, v=2.4.

The cathode chamber K_(K) also comprises an inlet Z_(KK) and an outletA_(KK) that enables addition of liquid, for example the solution L₂, tothe cathode chamber K_(K) and removal of liquid present therein, forexample the solution L₁. The inlet Z_(KK) and the outlet A_(KK) aremounted on the cathode chamber K_(K) in such a way that the solutioncomes into contact with the cathodic electrode E_(K) as it flows throughthe cathode chamber K_(K). This is a prerequisite for the solution L₁being obtained at the outlet A_(KK) in the performance of the processaccording to the invention when the solution L₂ of an alkali metalalkoxide XOR in the alcohol ROH is routed through K_(K).

The anode chamber K_(A) also comprises an outlet A_(KA) that enablesremoval of liquid present in the anode chamber K_(A), for example theaqueous solution L₄. In addition, the middle chamber K_(M) comprises aninlet Z_(KM), while K_(A) and K_(M) are connected to one another by aconnection V_(AM). As a result, it is possible to add a solution L₃ toK_(M) and then route it through K_(M), and then to route it via V_(AM)into the anode chamber K_(A), then through this K_(A). V_(AM) and theoutlet A_(KA) are mounted on the anode chamber K_(A) in such a way thatthe solution L₃ comes into contact with the anodic electrode E_(A) as itflows through the anode chamber K_(A). This is a prerequisite for theaqueous solution L₄ being obtained at the outlet A_(KA) in theperformance of the process according to the invention when the solutionL₃ is routed first through K_(M), then via V_(AM), then through K_(A).

Inlets Z_(KK), Z_(KM), Z_(KA) and outlets A_(KK), A_(KA), A_(KM) may bemounted on the electrolysis cell by methods known to the person skilledin the art.

According to the invention, the middle chamber K_(M) has a gas outlet G<120>. A suitable gas outlet is any opening in the middle chamber K_(M)through which gases formed in the middle chamber, especially CO₂, canescape into the atmosphere. The gas outlet is preferably mounted on themiddle chamber K_(M) in such a way that the CO₂ formed in the middlechamber K_(M) can escape from the middle chamber K_(M) counter togravity. This preferably takes place via a gas exit, preferably a gasexit with condensate separator, a pressure valve or a hole with aconnected vent conduit, more preferably through a hole with connectedvent conduit.

The vent conduit is connected to the outlet A_(KA) <106> (i.e. thechlorine draw or the brine outlet from the anolyte chamber), but mayalternatively be operated with free discharge to the atmosphere.Utilizing the chlorine draw for the removal of the CO₂ has the advantageof a simplified construction of the electrolysis cell E. But the removalof the CO₂ independently of the chlorine draw leads to an improvement inthe chlorine composition since CO₂ is not diluted with the chlorine. Itis thus particularly preferable when the vent conduit is not connectedto the outlet A_(KA) <106> (i.e. the chlorine draw or the brine outletfrom the anolyte chamber), i.e. is operated with free discharge to theatmosphere.

The connection V_(AM) may be formed within the electrolysis cell Eand/or outside the electrolysis cell E,

If the connection V_(AM) is formed within the electrolysis cell E, it ispreferably formed by at least one perforation in the diffusion barrierD.

If the connection V_(AM) is formed outside the electrolysis cell E, itis preferably formed by a connection of K_(M) and K_(A) that runsoutside the electrolysis cell E, especially in that an outlet A_(KM) isformed in the middle chamber K_(M) through the outer wall W_(A),preferably at the base of the middle chamber K_(M), the inlet Z_(KM)more preferably being at the top end of the middle chamber K_(M), and aninlet Z_(KA) is formed in the anode chamber K_(A) through the outer wallW_(A), preferably at the base of the anode chamber K_(A), and these arepreferably connected by a conduit, for example a pipe or a hose,preferably comprising a material selected from rubber and plastic. Theoutlet A_(KA) is then more preferably at the top end of the anodechamber K_(A).

“Outlet A_(KM) at the base of the middle chamber K_(M)” means that theoutlet A_(KM) is mounted on the electrolysis cell E in such a way thatthe solution L₃ leaves the middle chamber K_(M) in the direction ofgravity.

“Inlet Z_(KA) at the base of the anode chamber K_(A)” means that theinlet Z_(KA) is mounted on the electrolysis cell E in such a way thatthe solution L₃ enters the anode chamber K_(A) counter to gravity.

“Inlet Z_(KM) at the top end of the middle chamber K_(M)” means that theinlet Z_(KM) is mounted on the electrolysis cell E in such a way thatthe solution L₃ enters the middle chamber K_(M) in the direction ofgravity.

“Outlet A_(KA) at the top end of the anode chamber K_(A)” means that theoutlet A_(KA) is mounted on the electrolysis cell E in such a way thatthe solution L₄ leaves the anode chamber K_(A) counter to gravity.

This embodiment is particularly advantageous and therefore preferredwhen the outlet A_(KM) is formed by the outer wall W_(A) at the base ofthe middle chamber K_(M), and the inlet Z_(KA) by the outer wall W_(A)at the base of the anode chamber K_(A). This arrangement makes itpossible in a particularly simple manner to separate gases formed in themiddle chamber K_(M) from L₃ via the gas outlet G, while gases formed inthe anode chamber K_(A) leave the anode chamber K_(A) with L₄ and canthen be separated off further.

Accordingly, the flow direction of L₃ into K_(M) is the opposite of orthe same as the flow direction of L₃ into K_(A), preferably theopposite, according to how the connection V_(AM) is mounted on theelectrolysis cell E. Preferably, the flow direction of L₃ into K_(M) isin the direction of gravity.

In a preferred embodiment of the present invention, connection V_(AM)between middle chamber K_(M) and anode chamber K_(A) is arranged suchthat at least part of the aqueous solution L₃, preferably the entireaqueous solution L₃, flows completely through the middle chamber K_(M)and the anode chamber K %.

When the connection V_(AM) <112> is formed outside the electrolysis cellE <100>, this may especially be implemented in that Z_(KM) <108> andA_(KM) <118> are arranged at opposite ends of the outer wall W_(A) <117>of the middle chamber K_(M) <103> (i.e. Z_(KM) <108> at the base andA_(KM) <118> at the top end of the electrolysis cell E <100> or viceversa, preferably vice versa) and Z_(KA) <119> and A_(KA)<106> arearranged at opposite ends of the outer wall W_(A) <117> of the anodechamber K_(A) <101> (i.e., as is preferred, Z_(KA) <119> at the base andA_(KA) <106> at the upper end of the electrolysis cell E <100> or viceversa), as shown more particularly in FIG. 1 . By virtue of thisgeometry. L₃ <114> must flow through the two chambers K_(M) <103> andK_(A) <101>. It is possible here for Z_(K) <119> and Z_(KM) <108> to beformed on the same side of the electrolysis cell E <100>, in which caseA_(KM) <118> and A_(K) <106> are automatically also formed on the sameside of the electrolysis cell E <100>. Alternatively, as shown in FIG. 1, it is possible for Z_(KA) <119> and Z_(KM) <108> to be formed onopposite sides of the electrolysis cell E <100>, in which case A_(KM)<118> and A_(KA) <106> are automatically also formed on opposite sidesof the electrolysis cell E <100>.

When the connection V_(AM) <112> is formed within the electrolysis cellE <100>, this may especially be implemented in that one side (“side A”)of the electrolysis cell E <100>, which is the top end or the base ofthe electrolysis cell E <100>, preferably the top end as shown in FIG. 2, comprises the inlet Z_(KM) <108> and the outlet A_(KA) <106>, and thediffusion barrier D <110> extends proceeding from this side A into theelectrolysis cell <100>, but does not quite reach up to the side (“sideB”) of the electrolysis cell E <100> opposite side A, which is then thebase or the top end of the electrolysis cell E <100>, and at the sametime covers 50% or more of the height of the three-chamber cell E <100>,preferably 60% to 99% of the height of the three-chamber cell E <100>,more preferably 70% to 95% of the height of the three-chamber cell E<100>, even more preferably 80% to 90% of the height of thethree-chamber cell E <100>, more preferably still 85% of the height ofthe three-chamber cell E <100>. Because the diffusion barrier D <110>does not touch side B of the three-chamber cell E <100>, a gap thusarises between diffusion barrier D <110> and the outer wall W_(A) ofside B of the three-chamber cell E <100>. In that case, the gap is theconnection V_(AM) <112>. By virtue of this geometry. L₃ must flowcompletely through the two chambers K_(M) and K_(A).

These embodiments best assure that the aqueous salt solution L₃ flowspast the acid-sensitive solid-state electrolyte before it comes intocontact with the anodic electrode E_(A) <104>, which results in theformation of acids.

Furthermore, it is especially preferable in these embodiments when thegas outlet G is mounted at the top end of the middle chamber K_(M),since this best assures that CO₂ only leaves the middle chamber, but aminimum amount of solution L₃.

According to the invention, “base of the electrolysis cell E” is theside of the electrolysis cell E through which a solution (e.g. L₃ <114>in the case of A_(KM) <118> in FIG. 1 ) exits from the electrolysis cellE in the direction of gravity, or the side of the electrolysis cell Ethrough which a solution (e.g. L₂ <113> in the case of Z_(KK) <107> inFIGS. 1 and 2 , and L₃ <114> in the case of A_(KA) <119> in FIG. 1 ) issupplied to the electrolysis cell E counter to gravity.

According to the invention, “top end of the electrolysis cell E” is theside of the electrolysis cell E through which a solution (e.g. L₄ <116>in the case of A_(KA) <106> and L₁ <115> in the case of A_(KK) <109> inFIGS. 1 and 2 ) exits from the electrolysis cell E counter to gravity,or the side of the electrolysis cell E through which a solution (e.g. L₃<114> in the case of Z_(KM) <108> in FIGS. 1 and 2 ) is supplied to theelectrolysis cell E in the direction of gravity.

Process Steps According to the Invention

The process according to the invention comprises steps (a), (b) and (c)as follows, which are performed simultaneously.

In step (a), a solution L₂ comprising the alcohol ROH, preferablycomprising a sodium alkoxide NaOR in the alcohol ROH, is routed throughK_(K). R is an alkyl radical having 1 to 4 carbon atoms.

R is preferably selected from the group consisting of n-propyl,iso-propyl, ethyl and methyl, more preferably from the group consistingof ethyl and methyl. R is most preferably methyl.

Solution L₂ is preferably free of water. What is meant in accordancewith the invention by “free of water” is that the weight of water insolution L₂ based on the weight of the alcohol ROH in solution L₂ (massratio) is ≤1:10, more preferably ≤1:20, even more preferably ≤1:100,even more preferably ≤0.5:100.

If solution L₂ comprises NaOR, the proportion by mass of NaOR insolution L₂, based on the overall solution L₂, is especially >0% to 30%by weight, preferably 5% to 20% by weight, more preferably 10% to 20% byweight, more preferably 10% to 15% by weight, most preferably 13% to 14%by weight, at the very most preferably 13% by weight.

If solution L₂ comprises NaOR, the mass ratio of NaOR to alcohol ROH insolution L₂ is especially in the range of 1:100 to 1:5, more preferablyin the range of 1:25 to 3:20, even more preferably in the range of 1:12to 1:8, even more preferably 1:10.

In step (b), a neutral or alkaline, aqueous solution L₃ comprising NaCland at least one salt S selected from hydrogencarbonate and carbonate isrouted through K_(M) <103>, then via V_(AM) <112>, then through K_(A)<101>.

The salt S is especially selected from sodium hydrogencarbonate andsodium carbonate.

The pH of the aqueous solution L₃ is ≥7.0, preferably in the range of 7to 12, more preferably in the range of 8 to 11, even more preferably 10to 11, most preferably 10.5.

The proportion by mass of NaCl in solution L₃ is preferably in the rangeof >0% to 20% by weight, preferably 1% to 20% by weight, more preferably5% to 20% by weight, even more preferably 10% to 20% by weight, mostpreferably 20% by weight, based on the overall solution L₃.

The proportion by mass of salt S in solution L₃ is preferably in therange of 0.1 ppm to 10% by weight, more preferably in the range of 1 ppmto 5% by weight, even more preferably 10 ppm to 1% by weight, even morepreferably still 20 ppm to 400 ppm, especially preferably 20 to 100 ppm.

In step (c), it is then possible to apply a voltage between E_(A) andE_(K). This results in transfer of current from the charge source to theanode, transfer of charge via ions to the cathode and ultimatelytransfer of current back to the charge source. The charge source isknown to the person skilled in the art and is typically a rectifier thatconverts alternating current to direct current and can generateparticular voltages via voltage transformers.

This leads in turn to the following consequences:

solution L₁ <115> is obtained at the outlet A_(KK) <109>, w % herein theconcentration of NaOR in L₁ <115> is higher than in L₂ <113>,

an aqueous solution L₄ <116> of NaCl is obtained at the outlet A_(KA)<106>, wherein the concentration of NaCl in L₄ <116> is lower than in L₃<114>, and wherein the total concentration of all salts S in L₄ <116> islower than in L₃ <114>,

CO₂ <121> forms in the middle chamber K_(M) <103>, which is removed fromthe middle chamber K_(M) <103> via the gas outlet G <120>.

In the process according to the invention, in particular, such a voltageis applied that such a current flows that the current density (=ratio ofthe current supplied to the electrolysis cell to the area of thesolid-state electrolyte in contact with the anolyte present in themiddle chamber K_(M)) is in the range from 10 to 8000 A/m², morepreferably in the range from 100 to 2000 A/m², even more preferably inthe range from 300 to 800 A/m², and most preferably is 363 A/m². Thiscan be determined in a standard manner by the person skilled in the art.The area of the solid-state electrolyte in contact with the anolytepresent in the middle chamber K_(M) is especially 0.0001 to 10 m²,preferably 0.001 to 2.5 m², more preferably 0.022 to 0.15 m², even morepreferably 0.022 to 0.03 m².

It will be apparent that, in the process according to the invention,step (c) is performed when the two chambers K_(M) and K_(A) are at leastpartly laden with L₃ and K_(K) is at least partly laden with L₂.

The fact that transfer of charge takes place between E_(A) and E_(K) instep (c) implies that K_(K), K_(M) and K_(A) are simultaneously ladenwith L₂ and L₃ such that they cover the electrodes E_(A) and E_(K) tosuch an extent that the circuit is complete.

This is the case especially when a liquid stream of L₃ is routedcontinuously through K_(M), V_(AM) and K_(A) and a liquid stream of L₂through K_(K), and the liquid stream of L₃ covers electrode E_(A) andthe liquid stream of L₂ covers electrode E_(K) at least partly,preferably completely.

In a further preferred embodiment, the process according to theinvention is performed continuously, i.e. step (a) and step (b) areperformed continuously, while applying voltage as per step (c).

During the performance of step (c), lowering of the pH in the middlechamber K_(M) in particular results in formation of CO₂, which isremoved from the middle chamber K_(M) <103> via the gas outlet G <120>,especially by free discharge to the atmosphere as described above.

After performance of step (c), solution L₁ is obtained at the outletA_(KK), wherein the concentration of NaOR in L₁ is higher than in L₂. IfL₂ already comprised NaOR, the concentration of NaOR in L₁ is preferably1.01 to 2.2 times, more preferably 1.04 to 1.8 times, even morepreferably 1.077 to 1.4 times, even more preferably 1.077 to 1.08 times,higher than in L₂, most preferably 1.077 times higher than in L₂, wherethe proportion by mass of XOR in L₁ and in L₂ is more preferably in therange from 10% to 20% by weight, even more preferably 13% to 14% byweight.

At the outlet A_(KA), an aqueous solution L₄ of NaCl and possibly also Sis obtained, where the total concentration of NaCl in L₄ is lower thanthat of NaCl in L₃.

The total concentration of all salts S in L₄ is lower than the totalconcentration of all salts S in L₃; more particularly, the proportion bymass of all salts S in L is 5% to 95% lower, preferably 10% to 90%lower, more preferably 20% to 80% lower, even more preferably 30% to 70%lower, even more preferably still 40% to 60% lower, yet more preferablystill 50% lower, than the proportion by mass of all salts S in L₃.

The concentration of sodium chloride in the aqueous solution L₃ ispreferably in the range of 3.5 to 5 mol/l, more preferably 4 mol/l. Theconcentration of sodium chloride in the aqueous solution L is morepreferably 0.5 mol/l lower than that of the aqueous solution L₃ used ineach case.

In particular, Cl₂ is also obtained at the outlet A_(KA). Thisdisproportionates according to the above equation (4) and preferablyoutgases in the anode chamber K_(A). It is typically removed from theelectrolysis cell E with the aqueous solution 14 at the outlet A_(KA).

In particular, the process according to the invention is performed at atemperature of 20° C. to 70° C., preferably 35° C. to 65° C., morepreferably 50° C. to 65° C., and a pressure of 0.5 bar to 1.5 bar,preferably 0.9 to 1.1 bar, more preferably 1.0 bar.

In the course of performance of the process according to the invention,hydrogen is typically formed in the cathode chamber K_(K), which can beremoved from the cell together with solution L₁ via outlet A_(KK). Themixture of hydrogen and solution L₁ can then, in a particular embodimentof the present invention, be separated by methods known to the personskilled in the art. In particular, chlorine or another halogen gas formsin the anode chamber K_(A), which can be removed from the cell togetherwith solution L₄ via outlet A_(KA). Additionally formed in the anodechamber K_(A) is especially also carbon dioxide, which can likewise beremoved. Chlorine and/or CO₂ may then, in a particular embodiment of thepresent invention, be separated from solution L₄ by methods known to theperson skilled in the art. It is then likewise possible, after the gaseshave been separated off, to separate the mixture of chlorine and CO₂from solution L₄ by methods known to the person skilled in the art.

It is preferable also to mount a further gas outlet G₂ in the anodechamber K_(A), with the aid of which CO₂ can be removed, optionally in amixture with chlorine, directly after formation in the anode chamberK_(A). The gas outlet G₂ is preferably mounted on the anode chamberK_(A) in such a way that the CO₂ formed in the anode chamber K_(A) canescape from the anode chamber K_(A) counter to gravity, optionallytogether with the Cl₂ formed during the electrolysis and/or any O₂formed.

These results were surprising and unexpected in the light of the priorart. The process according to the invention protects the acid-labilesolid-state electrolyte from corrosion without, as in the prior art,having to sacrifice alkoxide solution from the cathode space as buffersolution. Thus, the process according to the invention is more efficientthan the procedure described in WO 2008/076327 A1, in which the productsolution is used for the middle chamber, which reduces the overallconversion.

In addition, the formation of a gas bubble of CO₂ in the middle chamberK_(M) is reduced, and hence a more energy-efficient process is enabled.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a preferred embodiment of the invention in a three-chambercell E <100>. This comprises a cathode chamber K_(K) <102>, a middlechamber K_(M) <103> and an anode chamber K_(A) <101>. The anode chamberK_(A) <101> and the middle chamber K_(M) <103> are separated from oneanother by an anion exchange membrane as diffusion barrier D <110> thatextends over the entire cross section of the three-chamber cell E <100>.The cathode chamber K_(K) <102> and the middle chamber K_(M) <103> areseparated from one another by a permeable solid-state electrolyte(NaSICON) <111> selective for sodium ions, which extends over the entirecross section of the three-chamber cell E <100>. The cathode chamberK_(K) <102> comprises a cathodic electrode E_(K) <105>, an inlet Z_(KK)<107> and an outlet A_(KK) <109>.

The anode chamber K_(A) <101> comprises an anodic electrode E_(A) <104>and an outlet A_(KA) <106> and is connected to the middle chamber K_(M)<103> via the connection V_(AM) <112>. The middle chamber K_(M) <103>additionally comprises an inlet Z_(KM) <108> and a gas outlet G <120>.In the embodiment according to FIG. 1 , the connection V_(AM) <112> isformed outside the electrolysis cell E <100>, especially by a pipe orhose, the material of which may be selected from rubber and plastic,with which liquid can be routed from the middle chamber K_(M) <103> intothe anode chamber K_(A) <101> outside the outer wall W_(A) <117> of thethree-chamber cell E <100>. The connection V_(AM) <112> connects anoutlet A_(KM) <118> that penetrates the outer wall W_(A) <117> of theelectrolysis cell E <100> at the base of the middle chamber K_(M) <103>to an inlet Z_(KA) <119> that penetrates the outer wall W_(A) <117> ofthe electrolysis cell E <100> at the base of the anode chamber K_(A)<101>.

An electrolyte L₂ <113> is routed into the cathode chamber K_(K) <102>via the inlet Z_(KK) <107>. The electrolyte L₂ <113> comprises methanol;the electrolyte L₂ <113> used is preferably a methanolic solution ofsodium methoxide L₂ <113>.

At the same time, an aqueous NaCl solution L₃ <114> with pH 10.5 and atotal proportion by mass of sodium hydrogencarbonate and sodiumcarbonate totaling 20 to 100 ppm is introduced into the middle chamberK_(M) <103> via the inlet Z_(KM) <108>. This flows through the middlechamber K_(M) <103> and the connection V_(AM) <112> into the anodechamber K_(A) <101>.

At the same time, a voltage is applied between the cathodic electrodeE_(K) <105> and the anodic electrode E_(A)<104>. This results inreduction of methanol in the electrolyte L₂ <113> to give methoxide andH₂ in the cathode chamber K_(K) <102> (CH₃OH+e⁻→CH₃O⁻+½H₂). In the anodechamber K_(A) <101>, the oxidation of chloride ions takes place to givemolecular chlorine (Cl⁻→½Cl₂+e⁻). Chlorine gas (Cl₂) in water, accordingto the reaction Cl₂+H₂O→HOCl+HCl, forms hypochlorous acid andhydrochloric acid, which give an acidic reaction with further watermolecules. The acidity would damage the NaSICON solid-state electrolyte<111>, but is restricted to the anode chamber K_(A) <101> by thearrangement according to the invention, and hence kept away from theNaSICON solid-state electrolyte F_(K) <111> in the electrolysis cell E<100>. This considerably increases the lifetime thereof. This acidity isnevertheless sufficient to protonate HCO₃ ⁻ and CO₃ ²⁻ to give carbonicacid, which then, according to the equilibrium (5)

$\begin{matrix}\left. {H_{2}{CO}_{3}}\leftrightarrow{{CO}_{2} + {H_{2}O}} \right. & (5)\end{matrix}$outgases carbon dioxide <121>. This can escape from the middle chamberK_(M) <103> through the gas outlet G <121>.

At the same time, sodium ions diffuse from the middle chamber K_(M)<103> through the NaSICON solid-state electrolyte <111> into the cathodechamber K_(K) <102>. Overall, this increases the concentration of sodiummethoxide in the cathode chamber K_(K) <102>, which affords a methanolicsolution of sodium methoxide L₁ <115>, the sodium methoxideconcentration of which is elevated compared to L₂ <113>. Owing to thegeometry of the three-chamber cell E <100> and the guiding of theaqueous solution L₃ <114> according to the invention, the acid-sensitiveNaSICON solid-state electrolyte <111> is protected from the elevatedacidity, compared to L₃ <114>, of the solution L₄ <116> that results inthe anode chamber K_(A) <101>.

At the same time, chlorine gas forms at the anode E_(A)<104> in theanode chamber K_(A) <101>, which is removed via the outlet A_(KA) <106>together with solution L₄ <116>.

The embodiment of the present invention shown in FIG. 2 corresponds tothat shown in FIG. 1 . The only difference here is that the connectionV_(AM) <112> within the electrolysis cell E <100> takes such a form thatthe diffusion barrier D <110> does not extend over the entire crosssection of the three-chamber cell E <100>. The connection V_(AM) <112>from the middle chamber K_(M) <103> to the anode chamber K_(A) <101> isthus formed by a gap in the diffusion barrier D <110>. In furtherpreferred embodiments of the present invention, it is also possible toutilize diffusion barriers D <110> having more than one gap, such thatthe connection V_(AM) <112> between middle chamber K_(M) <103> and anodechamber K_(A) <101> is formed by multiple gaps.

EXAMPLES Comparative Example 1

Sodium methoxide (SM) was prepared via a cathodic process, wherein theanolyte supplied in the middle chamber is 20% by weight NaCl solution(in water) and that supplied in the cathode chamber is 10% by weightmethanolic SM solution. The NaCl solution comprised 0.04% by weight ofNa₂CO₃.

The electrolysis cell consisted of three chambers, as shown in FIG. 1 ,and the anolyte was transferred through the middle chamber into theanode chamber. The connection between middle chamber and anode chamberwas established by a hose mounted at the base of the electrolysis cell.The anode chamber and middle chamber were separated by a 0.03 m² cationexchange membrane (Asai Kasai, F 6801, polytetrafluoroethylene, withsulfonate groups and carboxylate groups). Cathode chamber and middlechamber were separated by a ceramic of the NaSICON type with an area of0.022 m². The ceramic has a chemical composition of the formulaNa_(3.4)Zr_(2.0)Si_(2.4)P_(0.6)O₁₂. The flow rate of the anolyte was 1l/h, that of the catholyte was 3.5 l/h, and a current of 8 A wasapplied. The temperature was 65° C. It was found that a gas bubble ofCO₂ forms in the middle chamber over a prolonged period, which isattributable to the carbonates in the brine. This leads to a rise in thevoltage.

The progression of voltage as the change in the initial voltage (in %)over time (in hours) is shown in FIG. 3 (♦).

Inventive Example

Comparative Example 1 was repeated, except that, as well as the inletfor the anolyte, a hole of diameter 2 mm was introduced into the middlechamber. A vent conduit was attached to this hole.

Comparative Example 1 was repeated with this electrolysis cell. Themeasured voltage over time corresponded to the voltage measured at thestart of Comparative Example 1. In contrast with Example 1, the voltagedoes not rise over the duration of the experiment (see FIG. 3 , ▪).

In addition, gas bubbles are clearly apparent, which escape through theadditional gas outlet. However, this prevents the formation of a CO₂bubble in the middle chamber.

Result

The use of a three-chamber cell as in the process according to theinvention prevents the corrosion of the solid-state electrolyte, and atthe same time there is no need to sacrifice alkali metal alkoxideproduct for the middle chamber.

In addition, by virtue of the formation of a gas draw in the middlechamber, it is also possible to use brine that has been pretreated withcarbonates or hydrogencarbonates and hence contains them at least tosome degree in the electrolysis, without formation of a gas bubble inthe middle chamber of the electrolysis cell. In addition, the content ofCO₂ in the chlorine gas generated in the anode chamber is alsodistinctly reduced, which facilitates the subsequent separation of theCO₂ from chlorine.

The invention claimed is:
 1. A process for preparing a solution Li of asodium alkoxide NaOR in an alcohol ROI-1, in an electrolysis cell E,wherein E comprises at least one anode chamber K_(A), at least onecathode chamber K_(K), and at least one interposed middle chamber K_(M),the process comprising: (a) routing a solution L₂ comprising the alcoholROH through the at least one cathode chamber K_(K), (b) routing aneutral or alkaline, aqueous solution L₃ comprising NaCl and at leastone salt S selected from the group consisting of hydrogencarbonate andcarbonate, through the at least one interposed middle chamber K_(M),then via a connection V_(AM), through the at least one anode chamberK_(A), and (c) applying voltage between an anodic electrode E_(A) and acathodic electrode E_(K), wherein (a), (b), and (c) are performedsimultaneously, wherein the at least one anode chamber K_(A) comprisesthe anodic electrode E_(A) and an outlet A_(KA), wherein the at leastone cathode chamber K_(K) comprises the cathodic electrode E_(K), aninlet Z_(KK) and an outlet A_(KK), wherein the at least one interposedmiddle chamber K_(M) comprises an inlet Z_(KM) and a gas outlet G,wherein the at least one interposed middle chamber K_(M) is separatedfrom the at least one anode chamber K_(A) by a diffusion barrier D andis separated from the at least one cathode chamber K_(K) by a sodiumcation-conducting solid-state electrolyte F_(K), wherein the at leastone anode chamber K_(A) and the at least one interposed middle chamberK_(M) are connected to one another by the connection V_(AM) throughwhich liquid can be routed from the at least one interposed middlechamber K_(M) into the at least one anode chamber K_(A), wherein theprocess affords the solution L₁ at the outlet A_(KK), wherein theconcentration of NaOR in the solution L₁ is higher than in the solutionL₂, wherein the process affords an aqueous solution L₄ of NaCl at theoutlet A_(KA), wherein the concentration of NaCl in the solution L₄ islower than in the solution L₃, and wherein the total concentration ofall salts S in the solution L₁ is lower than in the solution L₃, whereinthe process forms CO₂ in the at least one interposed middle chamberK_(M), which is removed from the at least one interposed middle chamberK_(M) via the gas outlet G, and wherein R is an alkyl radical having 1to 4 carbon atoms.
 2. The process according to claim 1, wherein R isselected from the group consisting of methyl and ethyl.
 3. The processaccording to claim 1, wherein the diffusion barrier D is selected fromthe group consisting of cation-conducting membranes and anion-conductingmembranes.
 4. The process according to claim 3, wherein the diffusionbarrier D is a sodium cation-conducting membrane.
 5. The processaccording to claim 1, wherein a flow direction of the solution L₃ in themiddle chamber K_(M) is the opposite of the flow direction of thesolution L₃ in the anode chamber K_(A).
 6. The process according toclaim 1, wherein the connection V_(AM) is formed within and/or outsidethe electrolysis cell E.
 7. The process according to claim 1, whereinthe connection V_(AM) between middle chamber K_(M) and anode chamberK_(A) is arranged in such a way that at least a portion of the aqueoussolution L₃ flows completely through the middle chamber K_(M) and theanode chamber K_(A).
 8. The process according to claim 1, wherein thesodium cation-conducting solid-state electrolyte F_(K) has a structureof the formulaM^(I) _(1+2w+x−y+z)M^(II) _(w)M^(III) _(x)Zr^(IV) _(2−w−x−y)M^(V)_(y)(SiO₄)_(z)(PO₄)_(3−z), wherein M^(I) is selected from Na⁺ and Li⁺,M^(II) is a divalent metal cation, M^(III) is a trivalent metal cation,M^(V) is a pentavalent metal cation, and the Roman indices I, II, III,IV, V indicate the oxidation numbers in which the respective metalcations exist, and wherein w, x, y, and z are real numbers, wherein0≤x<2, 0≤y<2, 0≤w<2, and 0≤z<3, and wherein w, x, y, and z are chosensuch that 1+2w+x−y+z≥0 and 2−w−x−y≥0.
 9. The process according to claim1, wherein the solution L₂ comprises the alcohol ROH and the sodiumalkoxide NaOR.
 10. The process according to claim 9, wherein the massratio of the sodium alkoxide NaOR to the alcohol ROH in the solution L₂is in the range from 1:100 to 1:5.
 11. The process according to claim 9,wherein the concentration of NaOR in the solution L₁ is 1.01 to 2.2times higher than in the solution L₂.
 12. The process according to claim1, wherein the concentration of NaCl in the solution L₃ is in the rangefrom 3.5 to 5 mol/l.
 13. The process according to claim 1, wherein aproportion by mass of the salt S in the solution L₃ is in the range from0.1 ppm to 10% by weight.
 14. The process according to claim 1, whereinthe process is performed at a temperature of 20 to 70° C. and a pressureof 0.5 to 1.5 bar.